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As a structural engineer I suggest to have a construction joint at1/4 to1/3 rd of span for continuous beams/slabs (but not2/3 to3/4th). The point of zero Bending Moment also known as point of contraflexure which occurs at two points in between the supports of continuous beams and slabs. Point of zero shear is that point where is Shear Force is zero. At this point +ve BM is maximum. -Ve BM is maximum where SF changes its sign.
Agreed with Mr. Appi Reddy Kothapally answer ,but I want to add it is preferable to have a construction joint at1/4 to1/5th of span for a continuous beams/slabs
In some concrete pouring practices and codes, the construction joint is advised at1/5 th the span for the continuous beam or slab. In such case the concrete is left to flow without special preparations required where the construction joint is almost at the point of Zero moment.
In some other practices and codes, the construction joint is advised at the point of Zero shear which is the same point of maximum +ve bending moment. This point is almost at middle of the span for continuous beams or slabs. At such case the concrete should be kept vertical with rough surface. This is normally maintained by applying a suitable vertical mesh to support poured concrete at the point of construction joint which should be prepared and well supported before starting concrete pouring.
The2nd method for construction joint by maintaining vertical rough surface at point of Zero shear is more recommended by international reputed concrete codes.
The best Location of Construction Joint at zero shear and it is accepted at locations of low shear (middle third of the span), because that construction joint is look like a crack which make the contribution of concrete in shear shall be zero while the shear shall be carried by shear friction of top or bottom reinforcement. Unless the designer check shear is safe by shear friction, the construction joints shall be at zero shear or points of low shear. Some designers put detail of cross diagonal additional reinforcement at beams at location of construction joints to safe shear when the construction joints have some shear. At all the above description the construction joints have vertical form work.
In Egypt for small works, they don,t make make form work for construction joints and the concrete is poor because it is not compacted beside construction joints, so they make the construction joints at zero moment to make the concrete have no forces at that construction joints (compression forces due to moment). Also they worry from the shoring of the form work that may settle which make cracks in the concrete, so they make construction joints at1/5 the next span assuming that if the shoring is settle,1/5 the span can work as a cantilever without any problem.
Engineering wise construction joints shall be at zero shear or at weak shear but it must be at that case with form work (shuttering).
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